1. Field of the Invention
The present invention relates to a pulley width adjusting device for a belt-type continuously variable transmission used for automobiles, industrial machines and the like.
2. Description of the Related Art
Belt-type continuously variable transmissions are used in some automobiles, industrial machines and the like. Each belt-type continuously variable transmission includes a pulley provided on a rotation shaft on a driving side (primal pulley), another pulley provided on a rotation shaft on a driven side (secondary pulley) and an endless belt that is put across these pulleys.
Each of the above-described pulleys includes a fixed pulley disk that is faced to the rotation shaft and a movable pulley disk. The fixed pulley disk is integrated with the rotation shaft. The movable pulley disk is formed non-rotatable with regard to the rotation shaft but movable in the axial direction. The movable pulley disk is moved in the axial direction of the rotation shaft by an actuator. As it is moved, the distance between the fixed pulley disk and the movable pulley disk, that is, the pulley width, is changed.
In order to change the rotation speed rate between the rotation shaft on the driving side and that of the driven side, the pulley width on the driving side and the pulley width on the driven side are changed synchronously in directions opposite to each other. For example, in the case where the secondary pulley is changed in an accelerating side, the pulley width of the primary pulley is narrowed and the pulley width of the secondary pulley is widened. In this manner, the endless belt is moved to the outer circumferential side of the primary pulley and the endless belt is moved to the central portion of the secondary pulley.
A ball screw mechanism is used as a means for moving the movable pulley disk in the axial direction of the rotation shaft. The ball screw mechanism includes a screw cylinder provided on an outer circumference of the rotation shaft and a nut member engaged with the outer circumference of the screw cylinder via a plurality of balls. Each ball is contained between a screw groove formed in the outer circumference of the screw cylinder and a screw groove formed in an inner circumference of the nut member. The screw cylinder is fixed to a transmission case for use as a supporting member. The nut member is rotatable with respect to the movable pulley disk and movable in the axial direction of the rotation shaft together with the movable pulley disk.
When the nut member is rotated by a driving source such as a motor, the nut member is moved in the axial direction of the rotation shaft together with the movable pulley disk. As it is moved so, the pulley width between the movable pulley disk and the fixed pulley disk is changed.
The screw cylinder is placed via a bearing on the outer circumference of the rotation shaft to be coaxially therewith. An end portion of the screw cylinder is inserted with a pressure to a recess portion made in the transmission case together with the bearing. By this insertion with pressure, the screw cylinder is fixed to the transmission case. The rotation shaft that runs through an inner side of the screw cylinder can be rotated via the bearing.
As described above, the conventional technique has such a structure that an end portion of the screw cylinder is pushed into the recess portion formed in the transmission case. For this structure, a fit margin must be provided at the end portion of the screw cylinder for press-fit. Therefore, the structure of the screw cylinder becomes complicated and the size increases, which results in the increase in the production cost. Further, the screw cylinder must be inserted with a pressure into the recess portion together with the bearing while the end portion of the screw cylinder is combined with the bearing. Therefore, the assembly operation is laborious, and thus the assembly efficiency is undesirably decreased.
Accordingly, the object of the present invention is to provide a pulley width adjusting device for a continuously variable transmission, having a simple structure that can easily fix a screw cylinder to a support member such as a transmission case at a high efficiency.